1. Field of the Invention
The invention relates to a low-deformation support device of an optical element, in particular an end plate of a projection objective of a projection exposure machine for microlithography for the purpose of producing semiconductor components, in a mount, the optical element being connected to the mount at least partly via a bonded connection which is located between the adjacent circumferential walls of mount and optical element, and the mount being provided with at least three bearing elements which are distributed over the circumference and by means of which the optical element is laterally and axially supported.
2. Description of the Related Art
A low-deformation support or an optical element is known from U.S. Pat. No. 5,428,482, a mount having a flat outer ring on whose inside diameter three elements are constructed. The geometry of the flexible elements results from a slot which, starting from the centre of the outer ring, is guided into the outer ring and extended in a fashion concentric with the outer ring. This results in flexible elements which merge integrally at one end into the outer ring and can string freely at the other free end. An optical element can be fastened directly by a bonded connection at the three free ends of the flexible elements, the free ends of the flexible elements also being able, however, to merge integrally into an inner ring such that the latter is arranged concentrically inside the outer ring. The optical element is then fastened on this inner ring.
In a further design of such a support, the flexible elements can also be configured such that they merge integrally into the outer ring at both ends, and thereby form a type of leaf spring element. In this case, there is constructed in the middle of the leaf spring a bearing surface pointing towards the centre of the outer ring and on which the optical element is supported. The effect of the flexible elements here is that thermally induced differing expansions or the outer ring and the optical element are compensated. In this way, deformations which act on the outer ring are transmitted only weakly to the optical element.
A substantial disadvantage of the leaf spring in the transmission of deformations is that moments are transmitted due to its being clamped at both ends. For example, moments can be produced by corrugated deformation of the outer ring in the direction of an optical axis of the optical element and thereby be transmitted to the optical element. This effect is stronger the further the two clamping points of the leaf spring are situated apart from one another in the outer ring. Corrugated deformations of the optical element have a particularly disadvantageous effect on the imaging behaviour of high resolution optical systems such as, for example, projection objectives for microlithography.
Also known from EP 0 230 277 E1 is an optical module which has a mount and an optical element. The mount is constructed as a ring on whose inside diameter three radially resilient elements are provided. Each of these radially resilient elements has a bearing surface which is directed towards the centre of the mount. The optical element is fastened on the bearing surfaces via a bonded connection.
It is disadvantageous in this type of low-deformation support of the optical element that the resilient elements are of very stiff construction in an axial direction, and can thereby transmit production-induced corrugations of the mount to the optical element.
JP 202141270 A discloses an optical module which has two retaining rings or retaining elements between which an optical element is introduced or clamped. Or the side directed towards the optical element, the retaining elements each have three bearing surfaces or locating surfaces which form the contact surfaces to the optical element.
It is disadvantageous here that friction effects can occur at the respective contact sites in the event of changes in temperature and of different coefficients of expansion of the retaining rings and the optical elements. Jerky sliding associated therewith can lead to maladjustment of the optical element. Such a solution can therefore not be used for optical systems where stringent requirements must be placed on the positional stabilty of the optical elements.